Digital X-ray tomosynthesis is an imaging technique that enables three-dimensional imaging of a patient using a large-area digital detector typically used for conventional (single projection) radiography. A finite number of projection images over a limited angular range, typically between 20° and 40°, are acquired by varying the relative orientations of the x-ray tube, patient and detector. This is usually accomplished by either moving both the detector and x-ray source or by fixing the position of the detector (source) and moving the x-ray source (detector). Three-dimensional data is reconstructed from the captured projections in the form of a number of slices through the patient anatomy, each parallel to the detector plane. A consequence of limited angular scanning is that the in-depth resolution is much lower than the in-plane resolution of the reconstructed object.
Reconstruction of volumetric data from a tomosynthesis system requires knowledge of the underlying capture geometry, including the orientation and position of the detector, the movement and position of the source and potential patient motion. Precise geometric information of the imaging system (orientation of the X-ray detector, and locations of the X-ray tube and X-ray detector during 2D projection image acquisition) affect the image quality of the reconstructed images. Mismapping between object space and the acquired 2D projection images can degrade spatial resolution and lead to image artifacts (e.g., ring artifacts).
In a conventional tomosynthesis system, many of the geometric variables are known, as the detector position is precisely specified and the relationship between source and detector is also well established. For stationary imaging scanners, acquisition geometry is fixed by the mechanical coupling of source and detector, such as through a C-arm or other type of gantry arrangement. Calibration of this geometry is straightforward, using a calibration phantom prior to image acquisition.
For a bed-side tomosynthesis system using mobile radiography apparatus, however, the detector is mechanically uncoupled from the source. Thus, the capture geometry is not fixed by system mechanics and can be difficult to determine with the desired accuracy. Mobile x-ray imaging scanners used for tomosynthesis are designed for seriously ill patients who cannot walk to, or stand in front of, stationary imaging scanners having fixed geometry. Instead, in order to image these patients, the detector is often manually positioned under the bed-ridden patient. In this acquisition environment, without the benefit of mechanically fixed source-to-detector geometry, other approaches are needed in order to accurately determine the geometry information in real time.
There is a need for a calibration utility that is suited to estimate the geometry of a mobile radiography system used for tomosynthesis or other 3D volume imaging and that overcomes the aforementioned limitations.